I think there's currently already one DIV around all the comments, that looks like:

<divid="commentlist"style="display:block">

You could try to use that existing DIV.

If you need another DIV to wrap around all the comments, the only option is to modify the CommentView.ascx file in the User Controls folder. This file is used across all themes. Typically it's more ideal to only modify your own theme, but for
this situation, the only thing I can think of would to be modify this User Controls\CommentView.ascx file. The part where the comments are output is: